In recent years, as a bus communication system has become more sophisticated and devices connected to a bus communication system have increased in number and improved in performance, a system bus is being placed under increasing load. Under such circumstances, requests issued beyond the capacity of a system bus or a memory bus have led to cases where only high-priority initiators are serviced, causing low-priority initiators to hang. Therefore, to give low-priority initiators a chance to be serviced, methods such as round-robin and LRU (Least Recently Used) have been adopted to provide equal service to each initiator. However, even in a system where round-robin or LRU is adopted, there is a problem. The problem is that, if there occurs a case where each initiator simultaneously or alternately issues a plurality of requests, and each initiator is given the bus access alternately and issues a command equally, some initiators may always be denied access and obliged to perform retries because the target they wish to access is always busy processing a command from another initiator, and the initiators may never get a chance to have their commands accepted and thus hang.
Patent Literature 1 discloses a system wherein high-priority initiators, for each of which an access interval is fixed, and low-priority initiators, for each of which an access interval is not prescribed, access the same target. In the system, a threshold is set to the bus release period for the low-priority initiators, and thereby release of the bus in use is interrupted and the bus access is assigned to the high-priority initiators so that high-priority accesses are guaranteed.
Patent Literature 1: JP2003-281083A